1. Field of the Invention
The present invention relates to a wireless base station for forming an adaptive array antenna pattern using a plurality of antennas.
2. Description of the Related Art
Digital communications devices used in recent years transmit information using a digital information signal (baseband signal) to modulate a carrier wave, thereby making transmission more efficient.
When digital communication is performed, a plurality of users can occupy a single frequency simultaneously, by improving transmission speed and using time-division multiplexing. This enables frequency resources to be used more effectively. Space division multiplexing techniques, in which a plurality of users can each occupy a plurality of channels on a single frequency simultaneously through use of an adaptive array method, have been attracting particular attention of late.
An adaptive array method forms an adaptive directional pattern (known as an array antenna pattern) using a plurality of antennas, so that wireless waves can be directed only toward a user located in a specific direction. In an adaptive array apparatus that includes four wireless units, each including a transmission circuit, a reception circuit, and an antenna, the amplitude and the phase of (a) transmission signals transmitted by each of the transmission circuits, and (b) reception signals received by each of the reception circuits are adjusted respectively at transmission and reception. This enables appropriate directional patterns to be formed at transmission and reception. Such an adaptive array method is explained in more detail in Kukan Ryoiki Ni Okeru Tekioshingo Shori To Sono Ouyougijyutsu Ronbuntokushu (Adaptive Signal Processing and Applied Techniques in the Spatial Domain: Special Edition) in Denshi Tsushin Gakkai Ronbunshi (The Transactions of the Institute of Electronics, Information, and Communication Engineers (IEICE)) Vol. J75-B-II No. 11, November), so detailed explanation is omitted here.
A wireless base station using an adaptive array method can communicate simultaneously with a plurality of mobile stations. This is achieved by forming a different directional pattern for each of the plurality of mobile stations to multiplex the mobile stations simultaneously on a single frequency. This type of communication may be referred to as path division multiple access (PDMA), and is a space diversity technique. PDMA is described in Pasu Bunkatsu Tagen Setsuzuoku (PDMA) Ido Tsushin Hoshiki (The Path Division Multiple Access (PDMA) Mobile Radio Communication Systems) (IEICE Technical Report RSC93-84 (1994-01), pages 37 to 44), so a detailed explanation is omitted here.
When an adaptive array wireless base station is used in a mobile communication system such as the personal handyphone system (PHS), use of directional patterns needs to be determined according to whether a mobile station is using a control channel used for controlling incoming and outgoing calls, or a traffic channel (also known as a communication channel) used for making calls. In other words, the wireless base station transmits and receives control signals on control channels using an omnidirectional pattern that does not use path multiplexing, and transmits and receives communication signals (audio signals) on traffic channels by path multiplexing using a directional pattern.
However, if a mobile communication system includes a plurality of wireless base stations using the related art technology described above, the problem of interference between control signals emitted by the plurality of base stations is inescapable.
To take one example, each wireless base station in the PHS transmits control signals intermittently in cycles of approximately 100 ms on a common frequency allocated for control channel use, by time-division multiplexing the common frequency with other wireless base stations. However, if interference from a control signal emitted by another wireless base station is experienced when a link channel establishing request is received from a mobile station, or when a link channel allocation (frequency number and timeslot number of a traffic channel) is transmitted, the wireless base station has to make a further attempt (retry) to switch from the control channel to the traffic channel, and further failures to complete the transfer may also ensue.